1. Field of the Invention
This present invention relates to a reflecting type liquid crystal display device, inclusive of a translucent type, for carrying out display by reflecting ambient light or front light. More particularly, the invention relates to an STN (super-twist nematic) type liquid crystal display device.
2. Description of the Related Art
Technical features required of portable terminal apparatuses are low power consumption, light weight and compactness in size. Since a liquid crystal display device can be driven at a relatively low voltage, the liquid crystal display device can reduce electric power consumption. In particular, since a reflecting type liquid crystal display device utilizes ambient light, it does not need back-light and therefore realizes light weight and compactness in size easily. The reflecting type liquid crystal display device suitable for portable terminal devices can be divided into two types, i.e., one type using two polarizing plates and another type using one polarizing plate.
In the type using two polarizing plates, a polarizing plate provided with a reflecting plate is bonded to a surface of either one of a pair of light transmitting substrates, which surface is opposite to a liquid crystal layer which is sandwiched between the light transmitting substrates. On the other hand, light incident from the other substrate side passes through the other substrate, the liquid crystal layer and the one substrate in this order, are reflected by the polarizing plate provided with the reflecting plate, and outgo as the light passes again through the one substrate, the liquid crystal layer and the other substrate in this order. In this instance, the image becomes double due to parallax resulting from the thickness of the substrate, and visibility drops markedly. In a reflecting type color liquid crystal display device using micro-color filters, for example, the light passes through the color filters of different colors between the incoming route and the outgoing route due to the parallax, and color purity drops markedly.
In the type using one polarizing plate, the reflecting plate can be disposed inside the liquid crystal cell. Therefore, parallax resulting from the thickness of the substrates described above does not occur. The drop of visibility resulting from doubling of the image and the drop of color resulting from color mixture do not occur, either. Therefore, the type using one polarizing plate has become predominant in the reflecting type liquid crystal display devices. In the reflecting type liquid crystal display device which uses one polarizing plate, incident ambient light is converted to linearly polarized light by a polarizing plate 2, then to elliptically polarized light by phase difference plates 3 and 4, are modulated by an STN type liquid crystal layer 9 and are reflected by a reflecting layer 15, as shown in FIG. 1. The travel of reflected light is the reverse of incident light, namely, reflected light outgoes after being modulated , and is observed.
It is important in the liquid crystal display device to greatly modulate an intensity ratio of outgoing light in the full wavelength range of visible light by an applied voltage to a liquid crystal layer. In general, when reflected light is employed to display in white, a birefringence phase difference between reflecting means and a polarizing plate is set to xcex or 2/xcex and reflected light is converted to linearly polarized light by the reflecting means. And when reflected light is used for display in black, the birefringence phase difference between the reflecting means and the polarizing plate is set to xcex/4 or 3 xcex/4 and reflected light is converted to circularly polarized light by the reflecting means. Therefore, when a voltage is applied to the liquid crystal layer and the phase difference is modulated to xcex, or xcex/2 to xcex/4, or 3 xcex/4, in the full wavelength range, achromatic display can be accomplished with high contrast and high lightness.
However, the display operation of the STN type liquid crystal display device is carried out by virtue of birefringence and rotatory polarization of light. Moreover, the birefringence phase difference involves wavelength dispersion. For these reasons, it is difficult to obtain circularly polarized light and linearly polarized light with high accuracy in a visible light range of 400 nm to 800 nm. In the case of circularly polarized light, in particular, it is necessary to set not only the phase difference to xcex/4 or 3 xcex/4 but also to set the ratio of the major axis to the minor axis of an outgoing ellipse to 1:1. In other words, the angle of the absorption axis of the polarizing plate, xcex94n.d and the angle of the slow axis of each phase difference plate, xcex94n.d of the liquid crystal layer and the twist angle, must be optimized. Examples of them are disclosed in, for example, Japanese Unexamined Patent Publication JP-A 10-161110 (1998) and JP-A 10-170906 (1998).
According to the optical arrangement of the reflecting type STN type liquid crystal display devices using one polarizing plate, inclusive of the prior art technologies disclosed in JP-A 10-161110 and JP-A 10-170906, which have been proposed in the past, optimization of optical compensation is not complete. Therefore, optimization of the contrast, lightness and tone has not yet been achieved with good balance.
A mode for obtaining dark display under the state where liquid crystal molecules are aligned substantially parallel to the substrate surface, that is, NB (Normally Black) mode for obtaining black display without application of a voltage, can more easily compensate for dark display, can obtain more easily satisfactory dark display, and can therefore obtain more easily higher contrast than a mode for obtaining dark display while the liquid crystal molecules are kept upright from the substrate surface, that is, NW (Normally White) mode for obtaining white display without application of the voltage.
It is an object of the invention to provide an STN reflecting type liquid crystal display device using one polarizing plate, which allows to obtain achromatic display of high contrast and high lightness, and provide excellent visibility.
The invention provides a reflecting type liquid crystal display device of NB mode, comprising an STN type liquid crystal cell; a first phase difference plate, a second phase difference plate and a polarizing plate, the first phase difference plate, second phase difference plate and polarizing plate being disposed in this order on one surface of the STN type liquid crystal cell; and reflecting means disposed on the other surface of the STN type liquid crystal cell, wherein a product xcex94nLC.dLC of birefringence xcex94nLC and thickness dLC of a liquid crystal layer of the liquid crystal cell is selected from a range of 660 nm to 830 nm, a product xcex94n1.d1 of birefringence xcex94n1 and thickness d1 of the first phase difference plate is selected from a range of 120 nm to 240 nm, and a produce xcex94n2.d2 of birefringence xcex94n2 and thickness d2 of the second phase difference plate is selected from a range of 300 nm to 430 nm, and wherein in the case where a twist direction of a major axis of liquid crystal molecules of the liquid crystal layer of the liquid crystal cell from the reflecting means side to the first phase difference plate side is assumed to be a positive direction, a twist angle "khgr" of liquid crystal molecules from a major axis of the reflecting means side liquid crystal molecules of the liquid crystal layer of the liquid crystal cell to a major axis of the first phase difference plate side liquid crystal molecules of the liquid crystal layer of the liquid crystal cell is selected from a range of 220xc2x0 to 260xc2x0, and angle xcex8 from a major axis of the first phase difference plate side liquid crystal molecules of the liquid crystal layer of the liquid crystal cell to a slow axis of the first phase difference plate is selected from a range of xe2x88x92130xc2x0 to xe2x88x9275xc2x0, an angle xcfx86 from the slow axis of the first phase difference plate to a slow axis of the second phase difference plate is selected from a range of xe2x88x9220xc2x0 to xe2x88x9260xc2x0, and an angle xcfx86 from the slow axis of the second phase difference plate to an absorption axis of the polarizing plate is selected from a range of xe2x88x9215xc2x0 to xe2x88x9245xc2x0.
According to the invention, incident ambient light is converted to linearly polarized light by the polarizing plate, then to elliptically polarized light by the second and first phase difference plates, are modulated by the liquid crystal layer of the STN type liquid crystal cell, and are reflected by the reflecting means. The travel of reflected light is the reverse of the incident light, namely, reflected light outgoes after being modulated, and is observed. In the STN type reflecting type liquid crystal display device of the NB mode using one polarizing plate, the characteristics and design conditions of the optical members such as the polarizing plate, the first and second phase difference plates and the liquid crystal layer are optimized. In other words, the retardation xcex94nLC.dLC of the liquid crystal layer, the retardation xcex94n1.d1 of the first phase difference plate, the retardation xcex94n2.d2 of the second phase difference plate, the twist angle "khgr", the angle xcex8, the angle xcfx86 and the angle xcfx86 are optimized as described above. In consequence, high contrast and high lightness can be obtained while achromatic black display is maintained, and excellent visibility can be obtained.
Furthermore, the invention provides a reflecting type liquid crystal display device of NB mode, comprising an STN type liquid crystal cell; a first phase difference plate, a second phase difference plate and a polarizing plate, the first phase difference plate, second phase difference plate and polarizing plate being disposed in this order on one surface of the STN type liquid crystal cell; and reflecting means disposed on the other surface of the STN type liquid crystal cell, wherein a product xcex94nLC.dLC of birefringence xcex94nLC and thickness dLC of a liquid crystal layer of the liquid crystal cell is selected from a range of 750 nm to 850 nm, a product xcex94n1.d1 of birefringence xcex94n1 and thickness d1 of the first phase difference plate is selected from a range of 150 nm to 250 nm, and a product xcex94n2.d2 of birefringence xcex94n2 and thickness d2 of the second phase difference plate is selected from a range of 630 nm to 730 nm, and wherein in the case where a twist direction of a major axis of liquid crystal molecules of the liquid crystal layer of the liquid crystal cell from the reflecting means side to the first phase difference plate side is assumed to be a positive direction, a twist angle "khgr" of liquid crystal molecules from a major axis of the reflecting means side liquid crystal molecules of the liquid crystal layer of the liquid crystal cell to a major axis of the first phase difference plate side liquid crystal molecules of the liquid crystal layer of the liquid crystal cell is selected from a range of 220xc2x0 to 260xc2x0, an angle xcex8 from a major axis of the first phase difference plate side liquid crystal molecules of the liquid crystal layer of the liquid crystal cell to a slow axis of the first phase difference plate is selected from a range of xe2x88x92145xc2x0 to xe2x88x92110xc2x0, an angle xcfx86 from the slow axis of the first phase difference plate to a slow axis of the second phase difference plate is selected from a range of xe2x88x9225xc2x0 to xe2x88x9260xc2x0, and an angle xcfx86 from the slow axis of the second phase difference plate to an absorption axis of the polarizing plate is selected from a range of +10xc2x0 to +40xc2x0.
According to the invention, high contrast and high lightness can be obtained while achromatic black display is maintained, by optimizing the retardation xcex94nLC.dLC of the liquid crystal layer, the retardation xcex94n1.d1 of the first phase difference plate, the retardation xcex94n2.d2 of the second phase difference plate, the twist angle "khgr", the angle xcex8, the angle xcfx86 and the angle xcfx86 as described above. The color at the time of non-application of a voltage (background color) is approximate to pure black. Furthermore, matching with a three-dimensional phase difference plate can be improved and broad visual field angle can be acquired.
Furthermore the invention provides a reflecting type liquid crystal display device of NB mode, comprising an STN type liquid crystal cell; a first phase difference plate, a second phase difference plate and a polarizing plate, the first phase difference plate, second phase difference plate and polarizing plate being disposed in this order on one surface of the STN type liquid crystal cell; and reflecting means disposed on the other surface of the STN type liquid crystal cell, wherein a product xcex94nLC.dLC of birefringence xcex94nLC and thickness dLC of a liquid crystal layer of the liquid crystal cell is selected from a range of 800 nm to 900 nm, a produce of xcex94n1.d1 of birefringence xcex94n1 and thickness d1 of the first phase difference plate is selected from a range of 430 nm to 530 nm, and a product xcex94n2.d2 of birefringence xcex94n2 and thickness d2 of the second phase difference plate is selected from a range of 630 nm to 730 nm, and wherein in the case where a twist direction of a major axis of liquid crystal molecules of the light crystal layer of the liquid crystal cell from the reflecting means side to the first phase difference plate side is assumed to be a positive direction, a twist angle "khgr" of liquid crystal molecules from a major axis of the reflecting means side liquid crystal molecules of the liquid crystal layer of the liquid crystal cell to a major axis of the first phase difference plate side liquid crystal molecules of the liquid crystal layer of the liquid crystal cell is selected from a range of 220xc2x0 to 260xc2x0, an angle xcex8 from a major axis of the first phase difference plate side liquid crystal molecules of the liquid crystal layer of the liquid crystal cell to a slow axis of the first phase difference plate is selected from a range of xe2x88x9250xc2x0 to xe2x88x9290xc2x0, an angle xcfx86 from the slow axis of the first phase difference plate to a slow axis of the second phase difference plate is selected from a range of xe2x88x9240xc2x0 to xe2x88x9280xc2x0, and an angle xcfx86 from the slow axis of the second phase difference plate to an absorption axis of the polarizing plate is selected from a range of +10xc2x0 to +40xc2x0.
According to the invention, high contrast and high lightness can be obtained while achromatic black display is maintained, by optimizing the retardation xcex94nLC.dLC of the liquid crystal layer, the retardation xcex94n1.d1 of the first phase difference plate, the retardation xcex94n2.d2 of the second phase difference plate, the twist angle "khgr", the angle xcex8, the angle xcfx86 and the angle xcfx86 as described above.
Moreover the invention provides a reflecting type liquid crystal display device of NB mode, comprising an STN type liquid crystal cell; a first phase difference plate, a second phase difference plate and a polarizing plate, the first phase difference plate, second phase difference plate and polarizing plate being disposed in this order on one surface of the STN type liquid crystal cell; and reflecting means disposed on the other surface of the STN type liquid crystal cell, wherein a product xcex94nLC.dLC of birefringence xcex94nLC and thickness dLC of a liquid crystal layer of the liquid crystal cell is selected from a range of 820 nm to 920 nm, a product of xcex94n1.d1 of birefringence xcex94n1 and a thickness d1 of the first phase difference plate is selected from a range of 470 nm to 570 nm, and a product xcex94n2.d2 of birefringence xcex94n2 and thickness d2 of the second phase difference plate is selected from a range of 350 nm to 450 nm, and wherein in the case where a twist direction of a major axis of liquid crystal molecules of the liquid crystal layer of the liquid crystal cell from the reflecting means side to the first phase difference plate side is assumed to be a positive direction, a twist angle "khgr" of liquid crystal molecules from a major axis of the reflecting means side liquid crystal molecules of the liquid crystal layer of the crystal cell to a major axis of the first phase difference plate side liquid crystal molecules of the liquid crystal layer of the liquid crystal cell is selected from a range of 220xc2x0 to 260xc2x0, an angle xcex8 from a major axis of the first phase difference plate side liquid crystal molecules of the liquid crystal layer of the liquid crystal cell to a slow axis of the first phase difference plate is selected from a range of xe2x88x9270xc2x0 to xe2x88x92110xc2x0, an angle xcfx86 from the slow axis of the first phase difference plate to a slow axis of the second phase difference plate is selected from a range of xe2x88x9240xc2x0 to xe2x88x9280xc2x0, and an angle 100 from the slow axis of the second phase difference plate to an absorption axis of the polarizing plate is selected from a range of xe2x88x9210xc2x0 to xe2x88x9240xc2x0.
According to the invention, high contrast and high lightness can be obtained while achromatic black display is maintained, by optimizing the retardation xcex94nLC.dLC of the liquid crystal layer, the retardation xcex94n1.d1 of the first phase difference plate, the retardation xcex94n2.d2 of the second phase difference plate, the twist angle "khgr", the angle xcex8, the angle xcfx86 and the angle xcfx86 as described above.
Furthermore the invention provides a reflecting type liquid crystal display device of NB mode, comprising an STN type liquid crystal cell; a first phase difference plate, a second phase difference plate and a polarizing plate, the first phase difference plate, second phase difference plate and polarizing plate being disposed in this order on one surface of the STN type liquid crystal cell; and reflecting means disposed on the other surface of the STN type liquid crystal cell, wherein a product xcex94nLC.dLC of birefringence xcex94nLC and thickness dLC of a liquid crystal layer of the liquid crystal cell is selected from a range of 870 nm to 970 nm, a product xcex94n1.d1 of birefringence xcex94n1 and a thickness d1 of the first phase difference plate is selected from a range of 50 nm to 150 nm, and a product xcex94n2.d2 of birefringence xcex94n2 and thickness d2 of the second phase difference plate is selected from a range of 600 nm to 800 nm, and wherein in the case where a twist direction of a major axis of liquid crystal molecules of the liquid crystal layer of the liquid crystal cell from the reflecting means side to the first phase difference plate side is assumed to be a positive direction, a twist angle "khgr" of liquid crystal molecules from a major axis of the reflecting means side liquid crystal molecules of the liquid crystal layer of the liquid crystal cell to a major axis of the first phase difference plate side liquid crystal molecules of the liquid crystal layer of the liquid crystal cell is selected from a range of 220xc2x0 to 260xc2x0, an angle xcex8 from a major axis of the first phase difference plate side liquid crystal molecules of the liquid crystal layer of the liquid crystal cell to a slow axis of the first phase difference plate is selected from a range of xe2x88x9260xc2x0 to xe2x88x92105xc2x0, an angle xcfx86 from the slow axis of the first phase difference plate to a slow axis of the second phase difference plate is selected from a range of xe2x88x9210xc2x0 to xe2x88x9240xc2x0, and an angle xcfx86 from the slow axis of the second phase difference plate to an absorption axis of the polarizing plate is selected from a range of xe2x88x9215xc2x0 to xe2x88x9240xc2x0.
According to the invention, high contrast and high lightness can be obtained while achromatic black display is maintained, by optimizing the retardation xcex94nLC.dLC of the liquid crystal layer, the retardation xcex94n1.d1 of the first phase difference plate, the retardation xcex94n2.d2 of the second phase difference plate, the twist angle "khgr", the angle xcex8, the angle xcfx86 and the angle xcfx86 as described above.
In the invention described above, the first and second phase difference plates are laminated so that the slow axes thereof form with each other an angle within 90xc2x0 in a direction opposite to the twist direction of the liquid crystal molecules of the liquid crystal layer. In other words, the slow axis of the second phase difference plate is set to an angle within 90xc2x0 in a direction opposite to the twist direction from the major axis direction of the reflecting means side liquid crystal molecules of the liquid crystal layer of the liquid crystal cell to the major axis direction of the first phase difference plate side liquid crystal molecules of the liquid crystal layer, as viewed from the slow axis of the first phase difference plate. In this way, a compensation function for rotatory polarization of light can be provided.
A plurality of phase difference plates can be disposed. However, the increase of parameters makes it difficult to optimize contrast, lightness and color, and invites the increase of the cost of production. Therefore, two phase difference plates are preferably disposed as in the invention described above. Incidentally, though one phase difference plate can compensate for birefringence, it cannot compensate for rotatory polarization.
In the invention described above, the product xcex94nLC.dLC of the liquid crystal layer is set to at least 600 nm. When the product xcex94nLC.dLC of the liquid crystal layer is smaller than 600 nm, the change amount of the birefringence phase difference when a voltage is applied, is small and sufficient lightness cannot be obtained. Even though satisfactory black display can be obtained, white display with high lightness cannot be made.
The first and second phase difference plates may be each a uniaxial oriented film.
The reflecting means may be disposed inside the liquid crystal cell. The incident light passes through the polarizing plate, the second phase difference plate, the first phase difference plate and the STN type liquid crystal cell in this order and is reflected by the reflecting means. The reflected light travels in a direction opposite to that of the incident light, and outgoes. Since the liquid crystal cell is constituted by sandwiching the liquid crystal layer by a pair of light transmitting substrates, the incident light passes through the light transmitting substrate and reaches the reflecting means when the reflecting means is disposed outside the liquid crystal cell. The reflected light passes through the light transmitting substrate and reaches the liquid crystal layer. Therefore, the reflection factor drops due to the light transmitting substrate. Parallax occurs also depending on the thickness of the light transmitting substrate. Because the reflecting means is disposed inside the liquid cell in the invention, however, the drop of the reflection factor due to the light transmitting substrate does not occur, and parallax resulting from the thickness of the light transmitting substrate can be eliminated.
The reflecting type liquid crystal display device described above comprises light scattering means disposed on a surface of the liquid crystal cell on the first phase difference plate side, and the reflecting means described above may be mirror surface reflecting means. The incident light passes through the polarizing plate, the second phase difference plate, the first phase difference plate, the light scattering means and the STN type liquid crystal cell in this order and is reflected by the mirror surface reflecting means. The reflected light travels in reverse to outgo. The light scattering means disposed on the surface of the liquid crystal cell on the side of the first phase difference plate can scatter appropriately the normal reflection components of the reflected light by the mirror surface reflecting means in the direction of the visual field angle. In consequence, apparent lightness can be improved.
In the invention it is preferable that when the refractive indices of the second phase difference plate in the in-plane direction are ns and nf and the refractive index in the thickness-wise direction is nz, a relationship of ns greater than nz greater than nf or nz greater than ns greater than nf is satisfied.
According to the invention, the visual field angle characteristics with good balance can be obtained by using the second phase difference plate which satisfies the relationship of the refractive indices ns, nf and nz as described above.
In the invention it is preferable that when the refractive index of the second phase difference plate in the direction of the slow axis is ns, the refractive index in the direction of the fast axis is nf and the refractive index in the thickness-wise direction is nz, and as a parameter representing a magnitude of biaxial refraction is defined a coefficient Z as follows:
Z=(nsxe2x88x92nz)/(nsxe2x88x92nf), 
the coefficient Z is selected from a range of xe2x88x920.1 to 0.5.
The invention introduces the coefficient Z defined above and selects it from the range of xe2x88x920.1 to 0.5. Therefore, the invention can reliably obtain the visual field angle characteristics with good balance. While the uniaxially oriented two-dimensional phase difference plate compensates for the light incident from the vertical direction to the liquid crystal layer, the three-dimensional phase difference plate having such characteristics can compensate also for the light incident to the liquid crystal layer from oblique directions. Therefore, dependence of the incidence angle can be decreased and the visual field angle characteristics can be improved.
In the invention it is preferable that the reflecting type liquid crystal display device further comprises circularly polarized light selective irradiation means for selectively irradiating the circularly polarized light to the reflecting means from the side opposite to the liquid crystal cell of the reflecting means, and the reflecting means has a function of transmitting a part of light.
Circularly polarized light selected by the circularly polarized light selective irradiation means passes through the translucent type reflecting means, is modulated by the liquid crystal cell, is polarized to linearly polarized light or to elliptically polarized light by the first or second phase difference plate, and is absorbed by the polarizing plate, or a part of the light passes through the polarizing plate. Transmission type display can be accomplished by disposing a back-light, for example, besides the reflecting type display described above, and display can be made even when ambient light is weak or when it does not exist. Incidentally, the light incident from the side of the reflecting means passes only once through the liquid crystal layer and outgoes in the transmission type display. Therefore, circularly polarized light may be allowed to be incident in this case.
When the circularly polarized light selective irradiation means comprises a xcex/4 plate and a polarizing plate, the incident light is polarized to linearly polarized light by the polarizing plate constituting the circularly polarized selective irradiation means and, in the same way, to circularly polarized light by the xcex/4 plate constituting the circularly polarized selective irradiation means. Respective polarized light passes through the translucent type reflecting means, is modulated by the liquid crystal cell, is then converted to linearly polarized light or elliptically polarized light by the first or second phase difference plate, and is absorbed by the polarizing plate, or a part of light passes through the polarizing plate. In this way, display of the reflecting type and the transmission type can be accomplished.
When the circularly polarized light selective irradiation means comprises a cholesteric film, the cholesteric film constituting the circularly polarized light selective irradiation means selects right-turn light and left-turn light, In consequence, the incident light is converted to circularly polarized light, passes through the translucent reflecting means, is modulated by the liquid crystal cell, is then converted to linearly polarized light or elliptically polarized light by the first or second phase difference plate, and is absorbed by the polarizing plate, or a part of light passes through the polarizing plate. In this way, display of the reflecting type and the transmission type can be accomplished.
In the invention it is preferable that the liquid crystal cell is fabricated by sandwiching the liquid crystal layer between a pair of substrates, and plastic substrates are used as the substrates.
When the plastic substrate is used, for example, the invention makes it possible to reduce the weight of the liquid crystal display device and to improve its impact resistance.